1. Field of the Invention
The present invention relates to a production process for efficiently removing magnesium from a solution containing nickel and producing high-purity nickel sulfate, in particular, the invention can be applied to a treatment for an in-process intermediate product solution that is generated during a nickel hydrometallurgical process.
2. Description of the Related Art
A method for industrially producing nickel sulfate typically obtains a nickel sulfate solution, or nickel sulfate crystals by evaporation and crystallization or the like, through processes of dissolving a raw material in an acid solution and then removing impurities.
In the production method, the process of removing impurities may be carried out by various methods depending on the impurities included in the raw material. The removal process first carries out a solution purification process of forming a neutralized precipitate containing a portion of impurities and a residual liquid after separation using a neutralizing agent, and then carries out a solvent extraction process of extracting the residual liquid after separation using a conventional organic solvent to further perform a removal treatment of impurity elements.
Particularly, as a method for efficiently separating nickel and cobalt when cobalt is included in the raw material, a solvent extraction method using phosphonic acid or phosphinic acid has been widely known.
Regarding the phosphonic acid or phosphinic acid used in such a solvent extraction method, 2-ethylhexylphosphonic acid mono-2-ethylhexyl ester and di(2,4,4-trimethylpentyl)phosphinic acid are capable of satisfactory extraction and separation of nickel and cobalt and are thus suitable.
Furthermore, the solvent extraction using phosphonic acid and phosphinic acid is dependent on a pH of the solution, and the extraction efficiency is increased when the pH is increased. Also, since the extraction-related pH-dependency varies with elements, cobalt and other impurity elements are extracted into an organic solvent by utilizing this characteristic.
That is, impurity elements are divided into an organic phase by setting the pH to a value lower than pH at which nickel is extracted, and thus nickel remains in the aqueous phase, and as a result, a nickel solution after impurities have been removed can be obtained.
Furthermore, Japanese Patent Application Laid-Open No. 10-310437, Japanese Patent Application Laid-Open No. 10-30135 and Japanese Patent Application Laid-Open No. 2004-307270 disclose methods in which nickel is extracted in advance into an organic solvent under high pH conditions, this organic solvent containing extracted nickel is brought into contact with a nickel solution containing impurities, thereby an exchange reaction occurs by which elements that are more easily extracted than nickel are transferred to the organic phase, while nickel in the organic solvent is transferred to the aqueous phase side, and thus impurities in the nickel solution are removed.
These methods are effective as methods for preventing impurity elements such as Na included in a pH adjusting agent, from being incorporated into a nickel solution and contaminating a manufactured product.
However, among impurity elements, magnesium in the solution exhibits a reaction behavior similar to that of nickel, and therefore, it has been difficult to selectively remove magnesium from a nickel solution even if a solution purification process or a solvent extraction process was used in a process for producing nickel sulfate as described above.
For that reason, when a solution containing a small amount of nickel that is discharged from a solvent extraction process or a solution purification process (magnesium is also included in this solution) is recycled within the system, magnesium that remains unremoved also recurs, as is the case of nickel. Therefore, magnesium is accumulated in the system, and this has been a cause for an increase in the magnesium level in manufactured products.
Under such circumstances, an object of the present invention is to provide a removal method of an impurity element for selectively removing magnesium from a solution containing nickel, and a method for producing high-purity nickel sulfate by incorporating the removal method of an impurity element into a process of the production method.